complete: add SetExitFunc() to override os.Exit in tests

[charlievieth]

This eliminates the need to use the unsupported bou.ke/monkey package,
which fails to build on darwin/arm64. A better solution might be to remove
the use of os.Exit entirely since it's only called with os.Exit(0) and exiting
the program should be the responsibility of the caller to Complete.

[posener]

I think that this breaks the current package API. No?

[charlievieth]

@posener This change does not break the API or any existing code since all it does is add an additional function to the API for controlling how the complete library causes the program to exit.